Does conversion to conservation tillage really increase soil organic carbon stocks in organic arable farming?

Aggravation of weather extremes increases awareness of climate change consequences. Mitigation options are in demand which aim to reduce the atmospheric concentration of greenhouse gases. Amongst others, conversion from ploughing to conservation tillage is argued to increase soil organic carbon (SOC) stocks. Yet, main findings of reviews and meta-analyses comparing SOC stocks between tillage systems show different results: from a significant increase of SOC stocks to the question if there is any effect at all. Reasons are a sampling bias as in many campaigns only topsoil layers are assessed and horizons thickness is not considered adequately, different methods for SOC and bulk density determination, and the comparison of SOC stocks based on equivalent soil masses instead of equal sampling depths. In order to address these limitations, we initiated the SOCORT consortium (Soil Organic Carbon in Organic Reduced Tillage) – an international network of nine agronomical long-term trials. All trials represent common mixed organic farming systems of the respective region with organic fertilisation and crop rotations including leys. Climatic conditions are similar, but age and soil texture vary (7 to 21 years and sandy to clayey soils). A common sampling campaign was consequently elaborated to answer the question if the combination of conservation tillage and organic farming can really increase SOC stocks. Undisturbed soil cores were taken with driving hammer probes (8 cm in diameter) to a maximum depth of 100 cm. Each core was divided in the increments 0-30, 30-50, 50-70, 70-100 cm. The topsoil layer (0-30 cm) was further divided into the different tillage depths of the respective trial. All samples were analysed in the same laboratory for bulk density, organic carbon content, pH and texture. We compiled the yields for each trial to assess carbon inputs. The SOCORT consortium in combination with the common sampling campaign will entangle the driving factors of carbon sequestration through reduced tillage and add important knowledge on carbon dynamics in agro-ecosystems

Root-restricting layers in German agricultural soils. Part I: extent and cause

Aims Root-restricting layers pose a barrier to vertical root elongation. The German Agricultural Soil Inventory was used to assess the extent, cause and effect of root-restricting layers in German agricultural soils. Methods The following causes for root restriction were considered: bedrock, rock fragments, cementation, compactness, sandy subsoil, anoxia and acidity. Threshold values for restricted root growth were extracted from the literature and validated using root counts of winter wheat and permanent grassland. The effect of management-induced compaction in cropland was quantified using machine learning. Results In 71%of all agricultural soils, potential rooting was restricted to less than 100 cm depth. Compactness was the most common cause of root restriction, affecting 51% of cropland and 32% of grasslands. It was estimated that agricultural management explained 27% of all compacted cropland, while the remaining 73% has always been compacted as a result of pedogenic causes. Root-restricting soil layers decreased the yield of winter wheat significantly. Conclusions In view of potential rooting being restricted on more than half of Germany’s agricultural land and this study’s results suggesting that root-restricting soil layers have a direct impact on crop yield, there is considerable potential in the melioration of affected sites

Root-restricting layers in German agricultural soils. Part II: Adaptation and melioration strategies

Aims In many agricultural soils in Germany, elongation of deep roots is restricted by compactness, anoxia and acidity. This study examined the adaptation and melioration strategies of farmers who cultivate sites with such root-restricting layers (RRLs). Methods The German Agricultural Soil Inventory was evaluated with respect to land use and crop rotations on sites with and without RRLs. The likelihoods of deep tillage, drainage and liming and the feasibility of biological melioration (bio-drilling) were predicted using soil, geology, climate and socioeconomic data. Results Anoxic and acidic sites were preferentially used as grassland. Cropland with RRLs was often dominated by maize instead of wheat. About 54% of agricultural land in Germany was limed, 45% drained, 6% deep chiselled and 5% deep ploughed. The abundance of biopores was positively related to silt content and pH, but negatively related to rock content. Conclusions Deep tillage is not very popular for alleviating soil compactness, but bio-drilling could be used to facilitate deeper rooting in loamy and well-aerated soils with low rock fragment contents and pH values >5. Waterlogged soils could be meliorated by improved drainage and extreme acidity by enhanced liming practices. However, many farmers preferred grassland use as opposed to meliorating RRLs

CarboHedge – carbon sequestration via hedgerows

• A project to quantify the potential for CO2 sequestration through hedgerow replanting and the carbon stocks of hedgerows across Germany. • Hedges store around 140 tonnes more carbon per hectare compared to fields. • The majority (80%) of the additional carbon is stored in the biomass, but soil carbon stocks under the hedges are also increased

CO2-Zertifikate für die Festlegung atmosphärischen Kohlenstoffs in Böden: Methoden, Maßnahmen und Grenzen

Agrarböden besitzen durch den Aufbau von organsicher Bodensubstanz (Humus), die zu etwa 58% aus Kohlenstoff (Corg) besteht, ein großes Potential zur Kohlenstoffbindung. Positive Anstrengungen im Humusmanagement könnten daher einen wesentlichen Beitrag für den Klimaschutz leisten. Für Landwirtinnen und Landwirte stellen so genannte CO2-Zertifikate für den Aufbau von Corg („Humuszertifikate“) einen zusätzlichen Anreiz dar, humusfördernde Bewirtschaftungsmaßnahmen umzusetzen. Diese CO2-Zertifikate werden von privatwirtschaftlichen Initiativen und Unternehmen im Bereich des freiwilligen CO2-Markts vergeben. Insbesondere im Bereich der Landwirtschaft wächst im deutschsprachigen Raum derzeit der Zertifikatehandel für den Aufbau von Corg in Agrarböden. Um zum Klimaschutz beizutragen, müssen bei der Vergabe von Zertifikaten bestimmte Kriterien eingehalten werden. Wissenschaftliche Mindeststandards wurden dabei in der Praxis bislang jedoch wenig berücksichtigt. In dieser Studie werden Empfehlungen hinsichtlich der Erfassung von Corg (Probenahme, Analytik, Vorratsberechnung), eine Bewertung von Maßnahmen zum Corg-Aufbau, sowie Hinweise zu generellen Einschränkungen hinsichtlich des Klimaschutzes über CO2-Zertifikate gegeben. CO2-Zertifikate können einen positiven Anstoß geben, damit sich Landwirte verstärkt mit einer nachhaltigen Bewirtschaftung und Humusversorgung ihrer Böden auseinandersetzen. Da Humus die zentrale Steuergröße für viele Funktionen des Bodens und nicht zuletzt der Bodenfruchtbarkeit darstellt, ist jede Anstrengung für mehr Humus sinnvoll. Landwirtinnen und Landwirte, die sich für Humusaufbau interessieren, sollten daher hinsichtlich standort- und betriebsspezifischen Optionen zum Aufbau von Corg umfassend unterstützt und beraten werden

Carbon dynamics of young experimental afforestations in Thuringia

Nach Artikel 3.3. des Kyoto-Protokolls können Aufforstungen als Kohlenstoff (C) Senken angerechnet werden. Junge Aufforstungen können jedoch signifikante C-Quellen darstellen, wenn der C-Austrag durch Bodenatmung die C-Speicherung durch den Biomassezuwachs der Bäume übersteigt. Das Ziel dieser Arbeit war, i) die Faktoren zu untersuchen, die den Anwuchserfolg der Bäume bei Freiflächenaufforstungen bestimmen, ii) den Einfluss der Flächenvorbereitungsmaßnahmen und Veränderungen im Flächenmanagement auf die C-Bilanz des Systems zu quantifizieren, iii) den Einfluss historischer Landnutzungswechsel und der Bodenfauna (Regenwürmer) auf die C-Dynamik zu untersuchen und iv) die Heterogenität der C-Vorräte im Boden auf zwei Aufforstungsflächen zu analysieren, um ein optimiertes Beprobungsdesign für zukünftige Untersuchungen auf diesen Flächen zu entwickeln. Die Ergebnisse dieser Studien sind in sechs Publikationen zusammengefasst. Sie bilden die Grundlage für das Langzeitexperiment BIOTREE, in dessen Rahmen diese Arbeit angefertigt wurde. Das Experiment umfasst drei Flächen von insgesamt 70 ha in Thüringen. Das Ziel dieses Experimentes ist es in Zukunft den Einfluss der Baumartenvielfalt auf ökosystemare Prozesse zu untersuchen. Dazu wurden 300 000 Setzlinge 19 verschiedener Baumarten gepflanzt. Der Anwuchserfolg der 19 verschiedenen Baumarten, sowie die Einflussfaktoren, die den Anwuchserfolg bestimmen, wurde untersucht. Ausfälle von bis zu 79% pro Baumart verlängern die Zeit bis die Aufforstungsfläche zu einer Netto-C-Senke wird. Untersuchungsflächen mit hoher Baumartenvielfalt waren resistenter gegenüber Verbissschäden durch Schermäuse und Hasen als Flächen mit geringer Baumartenvielfalt. Die Flächen Mehrstedt und Kaltenborn wurden vor 23 und 29 Jahren teilweise von Acker zu Grünland umgewandelt. Diese vergangenen Landnutzungsänderungen ergaben keine signifikanten Veränderungen der C-Vorräte im Boden aber eine veränderte vertikale C-Verteilung. Hohe C-Vorräte wurden unterhalb der Pflugsohle auf den tonreichen Böden der Ackerfläche Mehrstedt gefunden. Die Quell- und Schrumpfdynamik der Tonminerale führte zu einem beschleunigten C-Transport in den Unterboden. 14C Altersbestimmungen des organischen Kohlenstoffs bestätigten diese Hypothese. Nur im obersten Bodenhorizont auf der Grünlandfläche Kaltenborn sind die mineralischen Oberflächen C-gesättigt und können deshalb keinen zusätzlichen Kohlenstoff physikalisch stabilisieren. Die großen ungesättigten mineralischen Oberflächen der Unterböden stellen ein ungenutztes Potenzial zur Stabilisierung und Speicherung von zusätzlichem Kohlenstoff dar. Der Netto-C-Fluss zwischen der Landoberfläche und der Atmosphäre wurde auf der Aufforstungsfläche Mehrstedt und einem angrenzendem Grünland mit zwei Eddy- Kovarianz-Türmen gemessen. Die Bruttoprimärproduktion der Aufforstungsfläche war um 41% (erstes Jahr) bis 14% (drittes Jahr) geringer als die der benachbarten Grünlandfläche. Die Flächenvorbereitung der Aufforstung mit Tieffräsen der Pflanzreihen zerstörte 30% der nicht-verholzten Vegetation, die die C-Flüsse der Fläche bestimmten. Eine beschleunigte Mineralisierung von Bodenkohlenstoff auf der Aufforstungsfläche führte im ersten Jahr zu einem Netto-C-Verlust von 1.2 t ha-1. Dahingegen war die saisonale C-Dynamik durch klimatische Faktoren bestimmt und durch Störungen durch das Flächenmanagement. Die Detektierbarkeit von Veränderungen der C-Vorräte im Boden wird durch deren räumliche und vertikale Heterogenität bestimmt. Die Variabilität der Bodenkohlenstoff-konzentration war ein bis zwei Größenordnungen größer als die der Feinbodendichte. Aus diesen beiden Parametern werden die C-Vorräte im Boden errechnet. Mit einem Simulationsmodel konnte gezeigt werden wie diese Information genutzt werden kann, um das Beprobungsdesign zu optimieren mit 12 - 19% weniger Proben aber unveränderter statistischer Genauigkeit. Der Einfluss von Regenwürmern auf den C-Transport und die C-Stabilisierung wurde untersucht, um den Effekt von verringerter Regenwurmabundanz in Wäldern auf die C Dynamik im Boden abschätzen zu können. Tiefgrabende Regenwürmer haben frischen Detritus schnell und effektive in den Unterboden transportiert und dort an den Gangwänden abgelagert. Entgegen der Hypothese, dass Regenwürmer zur C Stabilisierung beitragen wurde ein schneller C-Abbau in den Regenwurmgängen gemessen mit Umsatzzeiten von 3 bis 5 Jahren. Ein NMR (nuclear magnetic resonance) Relexationszeit-experiment und Messungen zur Enzymaktivität in den Regenwurmgängen ergaben keine Hinweise auf eine C-Stabilisierung durch Regenwürmer. Die C-Dynamik der untersuchten Aufforstungsflächen wird durch verschiedene Faktoren bestimmt, von denen sich einige kontinuierlich mit Heranwachsen des Waldes ändern werden, wie z.B. die Regenwurmabundanz oder die Bodenfeuchtedynamik. Dies wird zu Rückkopplungen auf den C-Kreislauf und auf die C-Speicherfunktion der Aufforstung führen.Afforestations are acknowledged as C sinks under the Kyoto protocol article 3.3. However, young afforestations may be considerable C sources. Losses of soil C may offset the C sink of the tree biomass. The aim of this thesis was to i) investigate the factors that affect the establishment success of the new forests, ii) quantify the impact of site preparation and management changes along with the afforestation on the C balance of the system, iii) understand how soil C dynamics are influenced by historical land use changes and activity of the soil fauna (earthworms), and iv) to explore soil C variability to set up an optimized sampling scheme for future soil C studies at the two afforestation sites. The essence of this research is presented in the form of six manuscripts. This thesis sets the basis for the long-term experiment BIOTREE which was started at three sites in Thuringia with a total of 70 ha. The future aim of this experiment is to investigate the influence of tree diversity on ecosystem processes. Therefore, 300 000 seedlings from 19 different tree species were planted. The design of the experiment is outlined in manuscript 1 together with a description of the three study sites. Manuscript 2 explores the differences between the establishment success of the tree species and the influencing factors. Establishment failure of the species up to 79% extends the time before afforestations become net C sinks. Experimental plots with higher tree diversity were found to be more resistant against damages by voles and rabbits than plots with less tree species. Parts of the sites Kaltenborn and Mehrstedt were converted from cropland to grassland, 23 and 29 years ago, respectively. The impact of this historical land use change on soil C stocks and C fractions was investigated (manuscript 3). Surprisingly, there was no significant difference in soil C stocks between both land use types but a different vertical C distribution was observed. High C stocks at the clay rich Mehrstedt site were found below the ploughing horizon. The swelling and shrinking dynamic of the clayey soil was expected to enhance the C transport into the subsoil. Measurements of the 14C age of this subsoil C confirmed this hypothesis. In the uppermost horizon of the sandy soil in the Kaltenborn grassland mineral surfaces were found to be C-saturated, thus, this horizon cannot physically stabilise additional C. The large area of unsaturated mineral surfaces in the subsoil provides an unused capacity to stabilise and store additional C at of both sites. Net C exchange fluxes between land surface and atmosphere were measured with two eddy covariance towers at the afforestation site Mehrstedt and an adjacent grassland site (manuscript 4). Gross primary productivity of the afforestation was reduced by 41% (first two years) to 14% (third year) compared to the grassland. Site preparation of the afforestation with deep ploughing damaged parts of the herbaceous vegetation that dominated the C fluxes. Enhanced C mineralisation was detected at the afforestation only during the first year, causing a net C loss of 1.2 t ha-1. Seasonal C dynamics were determined by climatic factors (mainly precipitation during summer) and disturbances by site management (grazing on grassland site, mowing on the afforestation site). The probability to detect expected soil C stock changes depends on the vertical and spatial heterogeneity of the C stocks. The variability of the soil C concentration was found to be one to two magnitudes higher than the variability of the bulk density. Both parameters directly affect the calculated soil C stocks. A simulation model revealed the possibility to improve the sampling design for soil C stocks with sample numbers reduced by 12-19% but unchanged statistical power. This is of major importance because high sample numbers are usually needed to make soil C stock changes detectable. The effect of earthworms on soil C translocation and stabilisation was investigated to understand how afforestations may influence the C cycling indirectly by reducing the earthworm abundance (manuscript 6). Deep burrowing earthworms were found to be effective in translocating recently assimilated C into the subsoil by depositing it along the burrow walls. Contrary to the original hypothesis of C stabilisation due to earthworm gut passage, organic C in earthworm burrows was lost rapidly with half life times of only 3 5 years. Nuclear magnetic resonance (NMR) relaxation experiments and enzyme activity measurements showed no enhanced C stabilisation by earthworms. The C dynamics of the investigated afforestation sites were found to be influenced by different factors. Some of them, such as earthworm abundance and seasonal soil moisture pattern, change along with the forest development feeding back on the C cycle and the C sequestration

Simulated wild boar bioturbation increases the stability of forest soil carbon

Most forest soils are characterised by a steep carbon gradient from the forest floor to the mineral soil, indicating that carbon is prevented from entry into the soil. Bioturbation can facilitate the incorporation of litter-derived carbon into the mineral soil. Wild boar are effective at mixing and grubbing in the soil and wild boar populations are increasing in many parts of the world. In a 6-year field study, we investigated the effect of simulated wild boar bioturbation on the stocks and stability of soil organic carbon in two forest areas. Regular bioturbation mimicking grubbing by wild boar was performed artificially in 23 plots, and the organic layer and mineral soil down to 15 cm depth were then sampled. No significant changes in soil organic carbon stocks were detected in the bioturbation plots compared with non-disturbed reference plots. However, around 50% of forest floor carbon was transferred with bioturbation to mineral soil carbon, and the stock of stabilised mineral-associated carbon increased by 28 %. Thus, a large proportion of the labile carbon in the forest floor was transformed into more stable carbon. Carbon saturation of mineral surfaces was not detected, but carbon loading per unit mineral surface increased by on average 66% in the forest floor due to bioturbation. This indicates that mineral forest soils have non-used capacity to stabilise and store carbon. Transfer of aboveground litter into the mineral soil is the only rate-limiting process. Wild boar may speed up this process with their grubbing activity

CarboHedge - CO2-Bindung durch Hecken

• Ein Projekt zur Quantifizierung des Potentials zur CO2 – Bindung durch Heckenneuanlage und der deutschlandweiten Kohlenstoffvorräte von Hecken. • Hecken speichern im Vergleich zu Äckern pro Hektar rund 140 Tonnen mehr Kohlenstoff. • Der Großteil (80%) des zusätzlichen Kohlenstoffs wird in der Biomasse gespeichert, aber auch die Bodenkohlenstoffvorräte unter den Hecken werden erhöht

Substrate quality of drained organic soils—Implications for carbon dioxide fluxes

Background: Peatlands only cover a minor fraction of the global terrestrial surface, but due to drainage, they are major contributors to carbon dioxide (CO2) emissions from soils. Previous studies have shown that hydrological conditions, nutrient availability and anthropogenic disturbance play an important role in the mineralisation of organic matter. Furthermore, microbial turnover depends on peat quality, which is determined by its botanical origin and degree of transformation under natural conditions. Aims: The objective of this study was to shed light on the interdependence between mineralisation rates, secondary transformation of peat and chemical composition by examining the differences between bog and fen peat and between strongly degraded topsoil and well-preserved subsoil. Methods: Bog and fen peat from ten different peatlands under grassland use in Germany were analysed for their chemical composition using standard 13C nuclear magnetic resonance (NMR) spectroscopy and wet chemical extractions for fibre analysis. The radiocarbon age was determined as well. The results were combined with CO2 fluxes from a previous incubation study. Results: Topsoils had higher shares of proteins and lipids, and lower shares of carbohydrates and aromatics than subsoils. Bog peat subsoils were characterised by higher shares of carbohydrates and lower shares of aromatics than fen peat subsoils. Topsoils were more similar to each other in their chemical composition than the subsoils. Considering all samples, aromatics and phenolics were negatively correlated with CO2 fluxes. Measured CO2 fluxes from topsoils were significantly higher than from subsoils. However, no influences of chemical composition on CO2 fluxes were detected when examining topsoils and subsoils separately. Even though aromatics and phenolics showed positive relationships with radiocarbon age, differences in age alone were unable to explain the higher amounts of these compounds in the subsoil. Conclusions: The results imply that chemical composition of topsoil peat is not the reason for higher mineralisation rates compared to subsoil peat, but rather a consequence of decomposition and transformation. Thus, peat mineralisation of drained organic soils under agriculture might not slow down over time due to gradually decreasing peat quality but could increase further

Stability of buried carbon in deep-ploughed forest and cropland soils - implications for carbon stocks

AbstractAccumulation of soil organic carbon (SOC) may play a key role in climate change mitigation and adaptation. In particular, subsoil provides a great potential for additional SOC storage due to the assumed higher stability of subsoil SOC. The fastest way in which SOC reaches the subsoil is via burial, e.g. via erosion or deep ploughing. We assessed the effect of active SOC burial through deep ploughing on long-term SOC stocks and stability in forest and cropland subsoil. After 25–48 years, deep-ploughed subsoil contained significantly more SOC than reference subsoils, in both forest soil (+48%) and cropland (+67%). However, total SOC stocks down to 100 cm in deep-ploughed soil were greater than in reference soil only in cropland, and not in forests. This was explained by slower SOC accumulation in topsoil of deep-ploughed forest soils. Buried SOC was on average 32% more stable than reference SOC, as revealed by long-term incubation. Moreover, buried subsoil SOC had higher apparent radiocarbon ages indicating that it is largely isolated from exchange with atmospheric CO2. We concluded that deep ploughing increased subsoil SOC storage and that the higher subsoil SOC stability is not only a result of selective preservation of more stable SOC fractions.</jats:p